Building System Models for RE
Chapter 10
Modeling Conceptual Objects
with Class Diagrams
Building models for RE
Chap.8: Goals
Chap.9: Risks
why ?
how ?
Chap.10: Conceptual objects
on what?
Chap.11: Agents
who ?
The object model



Structural view of the system being modeled (as-is or to-be)
Roughly, shows how relevant system concepts are structured
and interrelated
Represented by UML class diagram ...
– “objects”, classes not in the OO design sense: RE is concerned
with the problem world only !
– classes with no operations: data encapsulation is a design
concern; no design decisions here !

Multiple uses ...
– precise definition of system concepts involved in other views,
their structure & descriptive properties
– state variables manipulated in other views
– common vocabulary
– basis for generating a glossary of terms
Modeling conceptual objects: outline

What is a conceptual object?

Entities
BookCopy

Associations & multiplicities
Patron

Attributes
Patron
Name

Specialization
StudentPatron
Loan
BookCopy
Patron
StaffPatron

Aggregation

More on class diagrams
Library
BookCopy
– derived attributes, OR-associations, associations of associations

Building object models: heuristic rules
What is a conceptual object?

Set of instances of a system-specific concept ...
– distinctly identifiable
• immutable, built-in identity
• e.g. 2 string instances “Justine Henin” are the same,
but 2 Patron instances named Justine Henin are different
– can be enumerated in any system state
• in any state we can list all instances of the Patron concept
currently involved in the system
– share similar features
• common name, definition, type, domain properties,
• common attributes, associations: see details later
e.g. Email attrib of Patron; Loan assoc linking Patron and BookCopy
– may differ in their individual states and state transitions
Object instantiation:
classes & current instances

Every conceptual object has a built-in semantic relation
telling which instances are currently members of the object:
InstanceOf (o, Ob) iff o is currently an instance of Ob
– kept implicit in the object model, used for Def specification
– “current” state = some arbitrarily chosen system state
– e.g. InstanceOf (bc, BookCopy) says bc is currently member of
set BookCopy of book copies manimulated in the library system
– might not be the case 3 weeks earlier or 1 year later ...



A set of object instances may evolve over time
An instance may migrate from one object to another
– e.g. StudentPatron instance StaffPatron instance
An instance may be member of multiple objects
Object instantiation:
classes & current instances

(2)
Every concept in object model must be defined by Def annotation
specifying the necessary & sufficient condition for an individual to
satisfy InstanceOf (o, Ob)
– i.e. specific conditions for individual to appear & disappear as
instance of this object
– e.g. “ A patron is any person who has registered to the
corresponding library for the corresponding period of time
and has not been excluded since then“

When an individual becomes instance of an object, the object’s
attributes & associations get instantiated as state variables to
characterize it
e.g. InstanceOf (tr, Train)  tr.Speed, tr.DoorsState, On (tr, ...)

State variables of the system = set of state variables of all
conceptual objects declared in the object model
Types of conceptual object

Entity: autonomous, passive object
– instances may exist in system independently of instances of other
objects
– instances cannot control behavior of other objects
– e.g. Book, BookCopy ; Train, Platform, ...
– represented as UML class

Association: object dependent on objects it links
– instances are conceptual links among object instances
– e.g.
Loan linking Patron & BookCopy
Copy linking BookCopy & Book
At linking Train & Platform
On linking Train & Block
– represented as UML association
Types of conceptual object

(2)
Event: instantaneous object
– instances exist in single system state
InstanceOf (ev, Ev) denoted by Occurs (Ev)
– e.g. BookRequest ; StartTrain
– represented as UML class if attributes, associations needed

Agent: active, autonomous object
– instances have individual behavior =
sequence of state transitions for state variables they control
– e.g. Patron, Staff ; TrainController, TrainDriver
– represented as UML class if attributes, associations needed
O bject
S ubtype
E ntity
A ssociation
A ge nt
E vent
Object features as model annotations
a g en t
P a tro n
Nam e
fea tu res
E m a il
a ttribu tes
en tity
T ra in
Speed
A cc e le ra tio n
D o o rs S tate
N a m e T ra in ; T yp e E n tity
D e f A n y tra in c u rre n tly in o p e ra tio n in th e sys te m .
Has Speed % … % ;
A cc e le ra tio n % … % ;
D o o rs S ta te % … % ;
D o m In v a r
a n no tation
a sso cia tion
On
N a m e P atro n ; T yp e A g e n t
D e f A n y p e rs o n c u rre n tly re g is te re d to a s ystem ’s lib ra ry fo r s om e va lid p e rio d .
S yn o n ym s B o rro w e r
M o vin g IffN o n Z e ro S p e e d : ( tr: T ra in ) (M o vin g (tr)   tr.S p e e d  0 )
N o n Z e roS p e e d IfN o n Z e ro A c c e le r: ( tr: T ra in ) (tr.A cc e le ra tio n  0  tr.S p e e d  0 )
C los e d IfN o tO p e n : ( tr: T ra in ) (tr.D o o rs S ta te  ‘o p e n ’  tr.D o o rs S ta te = ‘c los e d ’)
In it F o r a n y tra in tr:
tr.S p e e d = 0 tr.A cc e le ra tio n = 0  tr.D o o rs S ta te = ‘c los e d ’
Is s u e H o w a b o u t tra ins u n d e r m a in te n a n c e ?
B lo ck
…
Nam e O n
D e f T h e c u rre n t lo c a liza tio n o f a tra in o n sys te m b loc ks .
D o m In v a r O n T w oB lo cks A tM os t: A tra in is o n o n e o r tw o s u cc e ss ive b lock s a t a n y tim e .
In it A n y tra in e n te rin g th e s ystem is o n th e m ain s ta tio n b loc k
Entities

As seen before: autonomous, passive object
– instances may exist in system independently of instances of other
objects
– distinctly identifiable, can be enumerated in any system state,
share similar features, may differ in individual states & transitions
– instances cannot control behavior of other objects
– characterized by Def, domain invariants, attributes, initialization

In the Def annotation, the conditions for an individual to appear &
disappear as instance of this entity must not necessarily refer to
other objects in the model
Library
Train
BookCopy
Block
Associations

Association = conceptual object linking other objects,
each playing specific role
– dependent on objects it links
– linked objects may be entities, associations, events, agents
roles
Train

holdsTrain
isOn
On
Block
Association instance = tuple of linked object instances,
each playing corresponding role
– may currently exist only if all instances are currently linked
and currently instances of corresponding objects

Predicate notation
Assoc (o1, ..., on) for InstanceOf ([o1, ..., on], Assoc)
Association instances

Association instance = tuple of linked object instances,
each playing corresponding role
Train
holdsTrain
isOn
On
Block
InstanceOf
tr1
tr2
On (tr2,bl1)
bl1
On (tr1,bl3)
bl2
bl3
Associations & their instances

Like for any object, association instances ...
– are distinctly identifiable
• built-in immutable identity =
tuple of identities of linked object instances
– can be enumerated in any system state
– are characterized by common features
• name, definition, attributes, domain invariants, initializations
– evolve individually from state to state ...
• instance appears as Ass (o1, ..., on) gets true (link creation)
• instance disappears as Ass (o1, ..., on) gets false (link deletion)
• state change as values of attached attrib, assoc are changing
Associations

(2)
Arity of association = number of objects linked by it
– Binary associations: arity = 2
– N-ary associations: arity > 2
• needed when links involving more than 2 objects must be
distinguished
• e.g. Registration linking ...
Patron (role MemberOf)
Library (role hasMember)
Period (role ValidityPeriod)
if binary, no distinction possible among registrations
of same patron & library for different periods

Reflexive association = same object appears under different roles
– e.g. Following linking Train (role Follows), Train (role FollowedBy)
Multiplicities of n-ary association

From fixed source (n-1)-tuple of currently linked instances:
min/max number of linked target instances
– attached to role of target instance

For binary associations, express standard constraints ...
– min = 0: optional link (possibly no link in some states)
– min = 1: mandatory link (at least one link to target in any state)
– max = 1: uniqueness (at most one link to target in any state)
– max = *: arbitrary number N of target instances linked to
source instance, in any state (N > 0)
Notation: “k” for “k..k”,
“*” for “0..*”
Borrows
BorrowedBy
Patron
O..1
Loan
O..Max
BookCopy
Entities, associations in UML
Command
association
*
Driving
Car
In
1
Train
a block may hold
0 or 1 train
0..1
isOn
*
On
1..2
Block
holdsTrain
At
Platform
0..1
entity
a train may be at
0 or 1 platform at most
Entities, agents, associations in UML
for a given libary and registration period,
there may be 0 up to an unbounded number
of registered patrons
Patron
agent
Loan
0..1
0..Max
Borrows
BorrowedBy
BookCopy
*
*
Period
0..1
1..*
Library
Registration
n-ary association
(2)
Copy
1
Book
Multiplicities, domain properties and goals

Multiplicities may encode some ...
– domain properties (descriptive)
"A train may be at one platform at most at a time"
– goals (prescriptive)
"A block may not accommodate more than one train at any time”
"A patron may not borrow more than Max book copies at a time”
to be found in the goal model as well !
=> source for goal elicitation (parent goals ?, subgoals ?)

BUT ...
– multiplicities mix prescriptive & descriptive assertions
– most assertions are not expressible by multiplicities
"A borrowed book must be returned within 2 weeks”
“A copy may not be both borrowed and available”
 need for other domain invariants, goals/requirements
Attributes

Intrinsic feature shared by any instance of an object
– entity, association, event, agent (like associations)

An attribute Att of object Ob is a function:
Att: Ob  SORT

Sort: set of possible attribute values (function range)
– NOT a conceptual object we want to model
– may be declared by ...
• predefined, domain-independent name
CopyAvailable: Boolean
PatronName: String
• domain-specific name
BlockSpeedLimit: Speed
• enumeration
GoSignal: {on, off}
Keywords: Topics
Attributes

(2)
Elementary attribute: sort is a set of atomic values
– e.g. DoorsState: {open, closed}

Structured attribute: sort defined with type constructor
– Tuple, SetOf, SequenceOf, Union
– e.g.


Keywords: SetOf [Topic] , dateRange: SeqOf [Date]
Precise, domain-specific semantics of attribute must be
defined in Has annotations
Attribute multiplicity: min/max number of values the attribute
may take
[0..x]: optional attribute
[x..*]: attribute value = set of values
[1..1]: mandatory attribute, single value: by default, omitted
e.g. PhoneNr [0..*]: String optional, possibly multiple values
Entities, associations, attributes in UML
Command
*
Driving
attribute
Car
In
CommandedSpeed: Speed
CommandedAccel : Acceleration
1
Train
0..1
isOn
CurrentSpeed: Speed
CurrentLoc: Location
DoorsState: {open,...}
*
On
0..1
holdsTrain
Block
SpeedLimit: Speed
At
Platform
0..1
...
Entities, agents, associations, attributes in UML
attribute of
association
Loan
DateBorrowed: Date
TimeLimit: NumberWeeks
DueReturnDate: Date
Patron
0..1
Borrows
0..Max BookCopy
BorrowedBy CopyID
Phone [*] : String
Period
*
Library
Copy
1
Book
Keywords [1..*] : Topics
Registration
DateRegistered: Date
Deposit: Money
attribute of
association
multiplicity
Modeling conceptual objects

What is a conceptual object?

Entities
BookCopy

Associations & multiplicities
Patron
Attributes
Patron
Name
Specialization
StudentPatron


Loan
BookCopy
Patron
StaffPatron

Aggregation

More on class diagrams

Building object models: heuristic rules
Library
BookCopy
Built-in associations
for structuring object models

Object specialization/generalization, decomposition/aggregation
– applicable to entities, agents, events, associations

Specialization = subclassing: object SubOb is a specialization of
object SuperOb iff for any individual o:
InstanceOf (o, SubOb)  InstanceOf (o, SuperOb)
• SubOb specializes SuperOb, SuperOb generalizes SubOb
• amounts to set inclusion on set of current instances

Feature inheritance as a consequence ...
– by default, SubOb inherits from SuperOb all its attributes,
associations, domain properties
• while have its own distinguishing features
– may be inhibited by compatible redefinition of feature with same
name within specialized SubOb (“override”)
Object specialization with inheritance
Command
*
Driving
Car
CommandedSpeed: Speed
CommandedAccel : Acceleration
1
Train
In
0..1
isOn
CurrentSpeed: Speed
CurrentLoc: Location
DoorsState: {open,...}
specialisation
Semi-rapid
*
On
0..1
holdsTrain
SpeedLimit: Speed
At
inherited features 0..1
Rapid
Block
Platform
...
Inhibiting inheritance
TrafficSignal
...
inherited
Color: {green, orange, red}
Location
WarningSignal
compatible
redefinition
(subsort)
Color: {orange}
The more specific feature always overrides the more general one
Multiple inheritance

Same object may be specialization of multiple super-objects
– by default, inheritance of all features from all super-objects

Can result in inheritance conflicts
– different features with same name inherited from different
super-objects
=> conflicting features first renamed to avoid this
renamed
StudentAddress
to avoid conflict
Patron
Address
Email
Student
Address
StudentID
StudentPatron
...
Multiple specializations

Same object may have multiple specializations
– Different subsets of object instances associated with different
criteria
– Same object instance may be member of different subsets (one
per criterion)

Discriminator = attribute of super-object whose values define
different specializations (differentiation criterion)
Tra in
d iscrim in a ting
a ttrib u te
Ac c eler ation
Door s State
Speed
S e m iR ap id
…
Capac ity
R a p id
…
S h o rtTra in
…
L o n g Train
…
Object generalization
Loan
featur es inher i ted by
all s pecia liz ations
DateBor r ow ed: D ate
Ti meLi mit: N um b erW eek s
…
r egis ter edAt
P a tro n
0..1
0..Max
Bor r owedBy
Phone [*]: Str i ng
Emai l: Pr efi x x Suffix Bor r ows
m ultiple
inher itanc e
S tu d e n tP a tro n
…
S tu d e n t
StudentID
…
Y ear Of Study
S ta ffP a tro n
Depar tment
B o rro w ab le Item
Copy ID
Date Enter ed
B o o kC op y Jo u rn alC op y
…
…
P ro ce e din gsC o p y
Res ear c hAcc ount
P ro cO f
Copy
…
Book
A uthor
Jo u rn al
Is s ue
C o n fe ren ce
Conf Ser ies
generalization is
not necessarily apparent
in problem world
Benefits of generalization-based structuring

Common features in multiple objects are factored out into
single generalized object
=> simpler model, no duplication

Generalized objects & their structure are reusable in
different contexts & systems (by specialization)
– e.g. BorrowableItem --> CDCopy , VideoCopy

Increased modifiability of large models
– modifications of more general features are localized in more
general objects, down-propagated to specialized objects
StudentPatron
NonPriviledgedPatron
Patron
Object aggregation

Aggregation = composite object whose components are objects
(“parts”)
InstanceOf (ob, AggrOb)  ob = Tuple (o1, ..., on)
with InstanceOf (oi , PartObi )
– applicable to entities, agents, events, associations
– multiplicities may be attached to part-to-aggregation links
– transitive, antisymmetrical links

Composition: aggregation & parts appear/disappear together
ob = Tuple (o1, ..., on) and not InstanceOf (ob, AggrOb)
 not InstanceOf (oi , PartObi )
– part object may then be part of one aggregation object only
Object aggregation: examples
L ib ra ry
Tra in
…
aggr egation
…
At
P la tfo rm
…
In
0..1
1..*
1
D ire cto ry
S h e lve
An tiTh e ft
Car
…
…
…
…
com pos ition
2
Door
Th e ftS e n so r
…
Th e ftAla rm
…
…
More on UML class diagrams

Derived attribute, association = defined in terms of other
attrib/assoc already in the model
– controlled form of redundancy
Loan
derived
attribute
DateBorrowed: Date
TimeLimit: NumberWeeks
/ DueReturnDate: Date
derived
association
Door
1..2
Car
In
Train
On
At
/ DoorsOf
Block
Platform
More on UML class diagrams

(2)
OR association = same role played by alternative objects
– set of object instances in this role =
union of alternative sets of object instances
O R-as s ociation
M as terO f
Book
0..1 …
B o rro w ab le Item
Copy ID
Date Enter ed
*
*
Copy
Copy
0..1
M as terO f
Jo u rn al
…
More on UML class diagrams

(3)
Ordered association: multiple target instances from source
instance (or tuple of instances) are ordered
Library
1
1..*
1
Directory
Shelve
0..1
(ordered)
BookCopy
CopyID
*
Content
0..1
AntiTheft
More on UML class diagrams

(4)
Association of associations: one of the linked objects is an
association
S eason
…
1 .. *
C o n c e rt
1
1
H a ll
…
…
P e rfo rm a n ce
D a te
1 .. *
R e s e rva tio n
1 .. *
R e s e rv#
R e s e rvD a te
Seat
S e a t#
Modeling conceptual objects

What is a conceptual object?

Entities
BookCopy

Associations & multiplicities
Patron
Attributes
Patron
Name
Specialization
StudentPatron


Loan
BookCopy
Patron
StaffPatron

Aggregation

More on class diagrams

Building object models: heuristic rules
Library
BookCopy
Building object models: heuristic rules

Deriving pertinent & complete object models from goal models
– deriving objects, associations, attributes
– introducing software-environment tracking associations
– identifying associations from domain invariants on multiple objects

Object or attribute ?

Entity, association, agent, or event ?

Attribute of a linked object or of a linking association ?

Aggregation or association ?

Specializing, generalizing concepts

Bad smells
Deriving objects, associations, attributes
from the goal model
F Review all specs of goals & domain properties in goal model ...
– take all referenced concepts meeting criteria for object,
and only those
• instances distinctly identifiable, enumerable in any state,
sharing similar features, differing in individual states
– consider the others as candidate qualifying attributes
• values are NOT concept instances to be characterized by
common attributes, associations
– identify associations + participating objects from linking
expressions in these specs
<sourceObj> <linkingVerb> <targetObj(s)>
<linkingNoun> of <targetObj> by <sourceObj>
Deriving objects, associations, attributes
from goal specs: example
Goal Maintain [BlockSpeedLimited]
Def The speed of a train on a block may never
exceed the limit associated with that block

Train
Speed: SpeedUnit
...
On
Block
SpeedLimit: SpeedUnit
...

Rephrasing sometimes needed to highlight linking expressions

Yet another reason for goal specs to be precise !
Deriving objects, associations, attributes
from goal specs: example (2)
Goal Maintain [WorstCaseStoppingDistance]
Def The distance between two trains following each other
shall be sufficient to prevent the back train from hitting
the front train in case the latter stops suddenly
Following
back
Train
front Speed: SpeedUnit
WCS-Dist: Distance
On
Block
SpeedLimit: SpeedUnit
...
Introducing software-environment
tracking associations
F For goal assignment to software-to-be, we must introduce
shared “images” of environment objects referenced by the goal
– the shared object tracking its environment counterpart must
accurately reflect it (=> new accuracy goal)
– e.g. TrainInfo (Speed, Position) tracking Train (Speed, Position)
LoanInfo tracking Loan, PatronInfo tracking Patron
– cf. goal refinement pattern seen before:
GoalOnUnMonitorableCondition
GoalOnUnControllableCondition
GoalOnMonitorable
Condition
MonitorableCondition Iff
UnMonitorableCondition
GoalOnControllable
Condition
ControllableCondition Iff
UnControllableCondition
Introducing software-environment
tracking associations:
a general pattern
Goal on environment Object
Goal onObj-Info
Software-to-be
Object Accurately
Tracked
By Obj-Info
tracks Obj-Info
Object trackedBy
1
Tracking 1
…
Att: Range
Refers to
Att: Range
Identifying associations from
domain invariants on multiple objects

Domain properties in goal refinements are to be defined in
annotations of the object model (as seen before)
– invariants on single objects they constrain

Invariant seeming to constrain multiple objects ...
? => ? constrains missing association among these ?
– e.g. “A platform cannot accommodate more than one train at a time”
... constrains platforms ? ... or trains ?
=> to be attached to missing association At linking
Train and Platform
Derivation links between
goal model & object model
are bidirectional

From goals to objects ...
– as just seen

From objects to goals ...
– Domain concepts that should “obviously” appear in object model ...
WHY? => missing goals in goal model
– Systematic association decoration with multiplicities ...
prescriptive? => missing goals in goal model
WHY? => parent goals
HOW? => subgoals
Object or attribute ?
F For X: conceptual item in goal specs,
make X an attribute if...
– X is a function: yielding one single value (possibly structured)
when applied to conceptual instance
– instances of X need not be distinguished
– you don’t want to attach attributes/associations to X, specialize
it, or aggregate/decompose it
– its range is not a concept you want to specialize or attach
attributes/associations
Book
Authors: String
Writing
Author
vs.
Birthdate
MainAuthor
Co-Author
Book
Entity, association, agent, or event ?
F For X: conceptual object in goal specs ...
– instances of X are defined in one single state
 event
e.g. StartTrain
– instances of X are active: control behaviors of other object instances
 agent
e.g. DoorsActuator
– instances of X are passive, autonomous
 entity
e.g. Train
– instances of X are passive, dependent on other, linked object
instances
 association e.g. Following (Train, Train)
F N-ary if each of the N parties ...
- need be considered as objects
- yields tuples to be distinguished
Attribute of a linked object or
of a linking association ?
Ob1
X?
0..
0..
Ob1
X?
Assoc
X?
F Attach attribute to association if
it explicitly or implicitly
characterizes all participating objects
– esp. if possibly no instance currently in some role,
to avoid losing info
– e.g. who did borrow this book copy?
Borrower
0..1
0..Max BookCopy
DateBorrowed
Loan
DateBorrowed
Aggregation or association ?
F For X a structural link between "composite" & "component"
objects, make it an association if any of these holds ...
– X has a domain-specific InstanceOf semantics
– component & composite objects seem independent
• component not subordinate to composite as in composition
– attributes or associations need be attached to the link type
– in case of doubt
e.g. In (Car, Train)
Specializing, generalizing concepts
F Identify specializations from classification expressions &
discriminant factors in goal/domprop specs
– taxonomical keywords “types of”, “kinds of”, “category”, “class”, ...
– meet object criteria ?
– relevant commonalities to factor out, specifics to discriminate ?
– multiple classifications under discriminating attributes ?
F Identify generalizations from objects characterized by similar
attributes, associations, domain invariants
– bottom-up search for common abstractions, not necessarily visible
– if worth doing so, without cluttering the model
StationBlock
Block
Corridor
N
Building object models: bad smells
F Avoid “pointers” to other objects as attributes
– use binary associations insead
BAD
BookCopy
Borrower
Loan: BookCopy
GOOD
Borrower
Loan
BookCopy
N
Building object models: bad smells
(2)
F Avoid non-structural links pertaining to other views
– monitoring/control links from agent model (context diagram)
TrainController
setsAcceleration
getsPosition
TrackingSystem
Initiator
setsDateRange
Meeting
Scheduler
setsDate
BAD
Train
Train
BAD
GOOD
BAD
Initiator
Initiating
Meeting
DateRange
Date
Scheduler
Scheduling
N
Building object models: bad smells
(3)
F Avoid non-structural links pertaining to other views
– dynamic links from behavior model (state diagram)
BAD
BorrowerRequest
Generates
Loan
BAD
GoSignal
Activates
Train
N
Building object models: bad smells
(4)
F Avoid obscure names for objects & attributes
– suggestive shortcut of their annotated definition
• don’t forget precise definition!
• don’t confuse terms !
e.g. Book vs. BookCopy
– from problem world, NOT implementation-oriented
Bad
JPEG_File , Book_File
Good Picture , Directory
– specific, NOT vague
Bad
Person , Form
Good
Patron , RegistrationForm
– commonly used, NOT invented
Bad
PersonalIdentificationCard, ConferenceBook
Good StudentCard, Proceedings